The present invention relates to data communications equipment, e.g., modems, and, more particularly, to cellular modem pools.
Generally speaking, a switched cellular data connection can be characterized as comprising two different types of communications channels. First, there is the cellular communications channel for coupling a cellular user to a Mobile Switching Center (MSC), and then there is a "land-line" communications channel, which is typically provided by the Public Switched Telephone Network (PSTN), for further coupling the cellular user to a PSTN-based data endpoint. As used herein, the term "land-line" refers to the remaining, i.e., non-cellular, communications channels provided by the PSTN, e.g., T1, local-loop, etc., whether using conducted transmission, e.g., a "tip-ring" pair, or radiated transmission, e.g., microwave transmission.
A cellular communications channel is sometimes referred to as an "impaired channel" since it is affected by a number of channel impairments like Rayleigh fading, co-channel interference, etc., that increase the error rate and, thus, degrade the overall performance of the mobile connection. This is in contrast to a land-line communications channel, where the predominant impairment is additive white gaussian noise (AWGN). As a result, those in the art have realized that when transmitting data in the cellular environment it is advantageous to use a "cellular modem pool" in the MSC to more effectively combat the different types of channel impairments presented by both the cellular and PSTN environments.
A cellular modem pool comprises a number of pairs of modems, in which the data terminal equipment (DTE) ports of each modem pair are cross-connected in a "back-to-back" fashion. This allows the two modems of each pair to interchange data via their DTE ports and thereby isolate that portion of the data connection over the cellular communications channel from that portion of the data connection through the PSTN. This isolation allows the use of a data protocol that is better suited to combating the effects of the cellular environment over the cellular portion of the data connection. One example of a cellular-oriented protocol is the "Enhanced Throughput Cellular" (ETC) protocol, developed by AT&T Paradyne. Similarly, on the PSTN side of the data connection, the cellular modem pool concept allows the modems at each endpoint of the PSTN connection to use a more traditional, e.g., V.42, land-line oriented protocol.
Typically, in order to establish a cellular data connection through a cellular modem pool, a cellular user dials both an access code and a telephone number associated with the called party. The access code informs the MSC to switch the incoming call through a modem pair of the cellular modem pool. Once the modem pair is switched in, the cellular side of the cellular modem pool negotiates, e.g., performs training, etc., with data communications equipment (DCE) of the cellular user. While the cellular portion of the data connection is being established, the PSTN portion of the data call is concurrently being setup, e.g., the telephone number of the called party is provided to the PSTN network.
Unfortunately, once the modem pair of the cellular modem pool is switched in, a cellular user can no longer hear the PSTN side "call progress" during the "data call setup" portion of the data call. As used herein, "data call setup" is that portion of a data call before data communications equipment of the called party provides answer tone. Consequently, if the PSTN side of the telephone call does not establish for some reason (for instance, if the number is busy, rings without answering, a wrong number is dialed and someone answers, etc.,) the cellular user has no idea what happened.